Dispersions with high pigment content provide a number of advantages to the printing ink manufacturer such as greater formulating latitude that permits the production of a greater variety of inks, cost reductions and lower inventory volumes.
The upper limit for the pigment concentration in a dispersion is determined by the resulting viscosity of the dispersion. As the pigment concentration increases linearly, the viscosity of the dispersion increases exponentially. However, if the viscosity becomes too high, the pigment concentrate can no longer be dispersed satisfactorily or easily handled during the ink manufacturing process. In addition, as the pigment concentration and resultant viscosity increases, the ink system experiences increasing heat build up due to frictional forces that occur during the dispersion process. This can be detrimental to the quality of ink as the increase in temperature may cause the degradation of certain pigments. Additionally, without proper control, lithography printing problems such as over-emulsification can occur, which is when the ink takes up excessive fountain solution causing poor ink transfer, blanket piling, high dot gain, misting; scumming, which is the inability of a fountain solution to keep the non-image area of a printing plate clean and is not easily washed from the plate; and feedback, which is the contamination of a fountain solution during the printing process that results in poor final print quality.
U.S. Pat. Nos. 4,859,247, 4,224,212, 4,057,436 and 3,996,059 describe oil-soluble viscosity reducing salts or cationic dispersant additive materials. Incorporation of these additives produces an increase in pigment concentration. However, their associated “surfactant-like” properties create a low interfacial tension and a very large a relative interfacial tension drop in oil-water ink systems which can result in lithographic printing problems (e.g. over-emulsification, scumming, or feedback). U.S. Pat. No. 6,290,768 discloses a pigment concentrate made up of pigment reacted with large amounts of natural resins or natural resin acids, monocyclic or polycyclic phenols, and small amounts of aldehydes or aldehyde acetals, Ia and IIa group metal compounds, and alpha, beta-ethylenically unsaturated carboxylic acids or their anhydrides. While the pigment dispersion prepared from such concentrate has good flow, it has high viscosity which can create ease of handling problems.
U.S. Pat. No. 5,935,315 discloses low viscosity aqueous pigment dispersions that are free of organic solvent and contain a fatty amidoalkoxylate dispersant. U.S. Pat. No. 5,882,393 discloses a dispersing agent or a salt of polyisocyanates reacted with a heterocyclic ring and a C2-C5 acrylate or epoxide moieties. Dispersants containing such salts exhibit poor lithographic printing performance. U.S. Pat. No. 5,645,636 discloses a pigment concentrated base and a dispersant of arylpararosanillnesulfonic acids derived from the condensation of an aliphatic amine or an ammonium compound and a polyester of hydroxyl fatty acid.
U.S. Pat. No. 5,688,312 discloses a hot melt ink composition containing polymeric imides or bisimides made by reacting a C2-C30 polyalkyleneoxide with a derivative of a phthalic anhydride or a succinic anhydride.
U.S. Pat. No. 5,336,314 and WO 02/34840 A1 describe dispersants, however neither of these dispersing vehicles can be used in a conventional flush systems employed to displace the dispersion vehicle of a pigment dispersion with another vehicle. U.S. Pat. No. 5,336,314 discloses reacting phthalic anhydride with a polyester polymer. WO 02/34840 A1 discloses a phthalic acid imide with a C30 maximum chain polyalkyl length for use in water-based systems. U.S. Pat. No. 5,429,841 discloses an ink containing a polyoxyalkylene substituted colorant.
U.S. Pat. No. 4,859,247 discloses an oil soluble viscosity reducing additive made by combining a long chain aliphatic amine and a polybasic organic acid in the form of salt. Incorporation of the additive allows increase pigment concentration in dispersions of up to 65 wt. %, but create significant problems in lithography printing such as scumming and feedback. U.S. Pat. No. 4,765,841 describes the preparation of organic pigment dispersions in non-aqueous medium directly from an aqueous slurry of the pigment. Dispersants are selected from resins, alkyds and mixed amide/salt-derivatives of polyesters derived from a hydroxycarboxylic acid. These dispersants can cause a substantial decrease in interfacial tension of an ink and the aforementioned scumming, feedback and over emulsification problems.
U.S. Pat. No. 4,468,255 discloses diazo and azo linkage Schiff's bases reacted with polyoxyalkane polymeric chains used as a rheological modifier for pigment dispersions. U.S. Pat. No. 4,461,647 discloses a non-aqueous pigment dispersant made of a diazo compound containing a single substituted ammonium-acid salt group. In general, salts create significant problems in lithography printing such as scumming and feedback.
U.S. Pat. No. 4,391,648 discloses a solid particulate granular pigment composition readily dispersible in an oleoresinous vehicle system made from a pigment, a cationic surfactant or blend of surfactants, and a water-soluble, non-crystalline, friable, thermoplastic polyester resin. Surfactants are placed in the dispersion which can cause scumming and feedback problems.
U.S. Pat. Nos. 4,340,431 and 4,309,320 describe the preparation of an organic pigment dispersion in a non-aqueous medium produced from an aqueous pigment slurry by incorporating an additive selected from sulfonated and sulfonated sperm oil.
However, the pigment loading is as low as 10 wt. %. U.S. Pat. No. 4,047,436 discloses a pigment dispersion using a quatemary ammonium salt of a monomeric colored acid. Quatemary ammonium compounds tend to cause the aforementioned lithographic printing problems and produce a low interfacial tension.
U.S. Pat. No. 4,028,128 describes a pigment dispersion containing a carboxy ester and pigment in organic liquids useful for paints, lacquers and gravure inks. However, the pigment concentrations are low. U.S. Pat. No. 3,966,059 discloses a pigment dispersion and a dispersion agent. These dispersions greatly suppress interfacial tension and can result in poor lithographic printing performance.
As indicated hereinabove, where the prior art discloses pigment dispersions having more than 45 wt. % pigment and viscosities of less than 150 Pa.s, said dispersions cause printing problems (e.g. over-emulsification, scumming, or feedback) in lithography printing. Offset inks are said to be over emulsified when excessive fountain solution emulsification causes poor ink transfer, blanket piling, high dot gain, and misting.
Surprisingly it has been found that careful control of interfacial tension, notably, maintaining a low-relative interfacial tension drop at the oil-water interface greatly helps to avoid known lithographic printing problems such as over-emulsification, scumming, and tinting.
Thus, there exists a need for high pigment load dispersions useful for formulating printing ink having low viscosity and low relative interfacial tension drop at the oil-water interface required for good lithographic printing performance and offering substantially reduced tendency toward scumming, feedback and over-emulsification.